Generally, alkenylnorbornenes (ANB) are commercially prepared via a Diels-Alder reaction employing cyclopentadiene (CPD) or its dimer dicyclopentadiene (DCPD), and an appropriate dienophile, for example, 1,3-butadiene (BD) where vinylnorbornene (VNB) is being prepared. As is known, the ANB product is formed as a mixture of its exo- and endo-isomers where the isomer ratio obtained for any such Diels-Alder reaction is a function of both the reaction temperature and the time the reaction mixture is held at such temperature as well as the specific reactants employed. Typical commercial VNB has been found to have an exo:endo ratio of about 1 to 3.
An analysis of the several products of a Diels-Alder reaction between BD and CPD make it possible to describe it as a set of parallel and consecutive reactions where the main products are VNB, tetrahydroindene (THI), 1,4-vinylcyclohexene (VCH), DCPD, and a mixture of several oligomeric compounds. The generation of VCH occurs either through direct dimerization of residual BD or when VNB undergoes a reverse-Diels-Alder reaction to yield BD and CPD where BD then dimerizes to yield VCH and, concomitantly, CPD dimerizes to generate dicyclopentadiene. It is believed that where other dienophiles are employed, an analogous set of parallel and consecutive reactions will result in analogous products.
Interest in obtaining the exo-isomer of VNB as well as the exo-isomers of other alkenylnorbornenes has grown recently as such isomers are promising building blocks in the synthesis of various biologically active compounds that exhibit diverse physiological activity. Additionally, it has been found that the exo and endo isomers of such monomers are different in their reactivity in polymerization processes and can provide polymers with different chemical and/or physical properties. However, as the separation of such endo- and exo-isomers by simple fractional distillation is extremely difficult, the use of individual isomers has been problematic. As VNB is the most common of the ANBs, the research directed to finding alternate methods of providing essentially pure, individual ANB isomers has focused on VNB isomers and in particular producing essentially pure exo-VNB. However, as shown below in the Comparative Example, and as there is a general absence of products that make use of the individual isomers, one might conclude that such research has not yet been successful.